Measurement of I-125 Brachytherapy Dose in Lung

Purpose: Sublobar resection and intraoperative placement of I-125 seeds along the staple line is an accepted treatment for patients with high risk stage I non-small cell lung cancer. The dosimetry standard for such an implant is to follow the TG43U1 protocol, which assigns water as the universal medium for dose calculation. This may not be appropriate for lung tissue. The purpose of this study is to characterize the dose distribution in lung relative to water using radiochromic film.

Methods: Seven Amersham 6711 I-125 seeds were sandwiched between slabs of Solid Water® on one side and lung-equivalent material on the other side. A sheet of Gafchromic EBT2 film was inserted parallel to the sources axes at the depth of measurement in the water and the lung simultaneously. Separate measurements were performed for three depths of 0.5, 1.0 and 1.5 cm. The seeds were far enough apart from each other so as not to influence the measurement from the neighboring seeds. The exposures lasted 2 hours, 5.5 hours and 15 hours for 0.5 cm, 1.0 cm and 1.5 cm, respectively. The total dose in water for these exposure times was 52 cGy, 35 cGy and 39 cGy, respectively. The ratios of lung over Solid Water® doses were averaged over the 7 seeds.

Results: The lung to Solid Water® dose ratio was 1.071±0.062, 1.205±0.090 and 1.259±0.053 for the depths of 0.5, 1.0 and 1.5 cm, respectively.

Conclusions: The dose in lung is quite similar to the dose in water at the depth of 0.5 cm, which is the prescription depth for a typical lung low dose rate brachytherapy implant. However, the attenuation in lung is smaller than in water, leading to an underestimation of lung dose at 1.5 cm by approximately 25% when TG43 parameters are used.